There is an increasing requirement for a high level of integration of electronic circuits, particularly involving different types of circuitry such as RF circuits, analogue baseband circuits and digital circuits. For example, there is a requirement for a mobile telephone or a television receiver integrated in a single monolithic chip, or in a single package containing more than one chip, to contain circuitry for implementing at least some of the following elements: a fast memory interface, various digital interfaces such as a Universal Serial Bus (USB) and a Inter-Integrated Circuit (IIC) bus, a digital processing unit, analogue and mixed signal blocks, a radio frequency (RF) receiver and transmitter, a power management unit, and peripheral interfaces to devices such as a display, camera and keyboard. Elements having an output with a high voltage swing and a high frequency, such as an RF transmitter, a DC to DC converter or a class D audio amplifier, can cause electrical interference to elements having a sensitive RF input, or a sensitive analogue input, such as a microphone input or a high resolution analogue to digital converter.
To some extent, such electrical interference can be reduced by careful positioning of input and output terminals on a chip or package. For example, a keyboard interface may have a low operating frequency and cause relatively little interference, and therefore the terminals for the keyboard interface may be positioned close to RF input terminals. However, even where a keyboard interface uses a low scanning frequency, high order harmonics may be emitted. To achieve a high performance in an RF receiver, the high frequency emissions coming from a keyboard interface need to be minimised.
Another problem is the RF interference a keyboard interface can experience if its terminals are placed close to an output of an integrated RF transmitter. A high power transmitter may induce spurious signals into high impedance inputs of a keyboard interface, which may cause an undesired operation. Therefore, there is a requirement for a keyboard interface that is insensitive to RF interference and which has low high frequency emissions.